Folding chassis for push-chairs or baby carriages are known which are essentially constituted by a lower frame, which comprises one or two front posts and one or two rear posts, which support in a lower region respective wheel assemblies, and by at least one upper post, known in the jargon as a handle tube, which terminates in an upper region in the grip or handle of the chassis and can move, with respect to the lower frame, with a movement that has a translational component and a rotary component, in order to allow the execution of the closing or compaction of the chassis.
With particular reference to what is called a clamshell- folding chassis, a typical constructive solution entails that the front and rear posts are mutually pivoted about an articulation axis, whereas the handle tube or tubes is/are kinematically connected to the lower frame by means of at least one linkage for connection to the respective rear posts, and by virtue of kinematic interconnection means constituted by at least one upper traction element that connects the handle tube or tubes to a front region of the rear post or posts and at least one lower fraction element that connects the rear post or posts to the upper post or posts.
This constructive solution allows, in the closing step of the chassis, to make the front posts approach the corresponding rear posts in an angular position while the handle tubes, thanks to a combined rotary and translational motion, arrange themselves, in the closed condition, substantially parallel to the rear posts with the handle arranged on the side opposite to the wheels.
Locking of the chassis in the open condition can be achieved by providing locking means that operate:                between the handle tube and the respective rear post in the contact region, —between the linkage and the handle tube,        between the handle tube and the upper traction element,        between the linkage and the rear post.        
The chassis has, during the transition from the extended open condition to the collapsed closed condition and vice versa, an equilibrium or “under fulcrum” point, determined by the distribution of the weights and the mutual arrangement of the relative rotation centers between the linkage and the handle tube, between the upper traction element and the handle tube and between the linkage and the rear post.
In practice, when the equilibrium point or subfulcrum point is passed, during the closure of the chassis, the weight of the chassis itself tends to facilitate the closure of the chassis, whereas during the opening of the chassis, when the equilibrium point or subfulcrum point is passed, the weight of the chassis tends to facilitate its opening.
However, it is clear that, during the initial step of the action intended to move the chassis from the extended condition to the collapsed condition, it is necessary, once the locking means have been deactivated, to force the movement of the handle tube in order to move toward the equilibrium point by means of an action that is not particularly practical and intuitive, since the user has to guide, by operating the grip, the handle tubes to perform a combined rotary and translational motion, in some cases even requiring the help of a foot.